The Effects of Moisture Content and Workhardening on Baseball Bat

The Effects of Moisture Content and Workhardening on Baseball Bat
The Effects of Moisture Content and Workhardening
on Baseball Bat Performance
P.J. Drane & J.A. Sherwood
Baseball Research Center, Department of Mechanical Engineering
University of Massachusetts Lowell, Lowell, Massachusetts USA
ABSTRACT: In recent years, baseball bat performance has come under scrutiny. Fans of
Major League Baseball have seen homerun records broken and the governing organizations
of both college and high school baseball have stepped in with tighter controls of baseball bat
performance. This paper discusses a few of the ways that baseball bat performance is
affected by changes in the material and physical properties of the bat. The scope of the
paper includes the effects of moisture content on the performance of solid wooden bats,
which are used for professional baseball in the United States, and the effects of
workhardening on the performance of aluminum bats, which are used by high school and
college players. The results from the moisture content tests of wooden bats showed that an
increase in the moisture content by soaking the bats in humidified air resulted in a small
increase in the batted-ball performance. Across the positions on the bat, which were tested,
the resulting increase reached a maximum of about 1%. The repeated hitting of an
aluminum bat with the intent of workhardening it also showed an increase in batted-ball
velocity of about 2%. Though, neither of these changes is enormous, they easily could be
the difference between the warning track fly-out and a homerun.
INTRODUCTION
It is believed that Abner Doubleday first developed the game of baseball in 1839. Though
the method of playing baseball has not significantly changed since, the equipment used has.
Most of the equipment changes have made the game safer for the players, e.g. batting
helmets, shin guards, face masks. Some changes have made the game more exciting, e.g.
the increasing of the liveliness of the baseball when Babe Ruth was playing. For the most
part these changes have been for the better by making the sport healthier for the players and
more exciting for the spectators. In recent years, some of these changes were beginning to
affect the game adversely, by increasing safety risks and leading to homerun derbies. The
introduction of high-performance aerospace-quality aluminum alloys and composites into
baseball bats used in many amateur leagues led the National Collegiate Athletic Association
(NCAA) to restrict the performance of the bats used in NCAA baseball in 2000. The NCAA
developed a testing protocol for measuring the performance of the baseball bat for
certification purposes. A summary of the NCAA testing protocol (1999) is shown in Fig. 1.
A schematic of the testing apparatus is shown in Fig. 2. The certification testing is
performed at the University of Massachusetts Lowell Baseball Research Center (UMLBRC).
Though professional baseball has never allowed baseball bats to be constructed of
materials other than solid wood, they have faced their own performance related stories. In
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1998, fans of Major League Baseball (MLB) were captivated by the contest between Sammy
Sosa and Mark McGuire to break the record of 61 homeruns in a season, which was intact
since 1961. The race ended with a new record of 70 homeruns in a season by McGuire. In
the 2001 season, the homerun record was again eclipsed with a 73-homerun performance by
Barry Bonds. Fans are beginning to question how a record that was unbreakable for 27
years is now being routinely broken.
To determine some of the characteristics ht at affect baseball bat performance, the
UMLBRC has conducted academic research in conjunction with MLB and NCAA. MLB is
concerned only with wooden bats because major league players are only allowed to use a bat
made of a solid piece of wood. The NCAA is concerned mainly with aluminum bats
because they are the preference for the majority of college players, and NCAA batting
statistics increased dramatically with the introduction of aluminum bats in 1974.
Summary of NCAA testing protocol
Environmental test conditions –
a. Relative Humidity - 50±15 %
b. Temperature - 75±10 ºF
2) Baseball bat speed - 66±1 mph (as measured at point 6 in from tip of barrel)
3) Baseball pitch speed - 70±2 mph
4) Batted-ball speeds measured at 9 inches, 13 inches, and 6 feet from impact
5) Valid hits must have
a. Baseball and Baseball bat speeds in required range
b. Either the 9-in or 13-in batted-ball velocity higher than the 6 foot
reading
c. Good targeting on trajectory
6a) Aluminum - 5 valid hits obtained at 5.0, 5.5, 6.0, 6.5, and 7.0-inch locations
from the barrel end of the bat
6b) Wooden – 3 to 5 valid hits at 5.5, 6.0, and 6.5-inch locations from the barrel end
of the bat
1)
Fig. 1 Summary of NCAA test protocol
Bat Swinging
Fixture
9 ft Speed Gate
Speed Gate
Sensors and
Emitters
6 ft Speed Gate
Hole for ball to
exit through.
Ball Swinging
Fixture
Protective
Enclosure
Servo-controlled
motors mounted
inside protective
boxes.
Fig. 2 Schematic of testing apparatus (Baum Hitting Machine)
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Bat and Ball
Swinging
Fixtures,
Speed Gate
Sensor and
Emiter Bar
EFFECTS OF MOISTURE CONTENT
Climates vary considerably across the United States. Additionally, the sport of baseball at
the major league level is played from the beginning of April to the end of October. The
springs in Boston are cool and wet, while the summers in Phoenix are hot and dry.
Different baseball stadiums will, therefore, expose wooden bats to different conditions at
different times of the year. The Wood Handbook (1999) published by the Forest Products
Society identifies the equilibrium moisture content for Phoenix, Arizona in the month of
June to be 4.6% on average, whereas Los Angeles, California is 15.1% in the month of
August. Though many baseball bats may not be exposed to the environment long enough to
come into equilibrium with these extremes, the bats which are stored in the environment for
even a few days will show a change in moisture content. It is, therefore, important to
determine the effects that a change in moisture content will have on wooden baseball bat
performance. Wood bats are typically kiln dried. During this drying process the cellular
structure of the wood changes due to chemical reactions. The simple forcing of moisture
back into the wood by soaking the bats in very humid air does not produce the same material
properties as would result from drying wood from its green state to that moisture level.
One method used for determining the effects of a change in moisture content of a
baseball bat is to test the same bat at two different levels of moisture content. Another
method uses different bats, which when tested, have the same physical properties, i.e. length,
weight, balance point, except for different moisture contents. The benefit of the first method
is that the variations in the properties of woods are essentially eliminated because both the
dry and the moist tests are performed on the same bat. This method requires a “swing
correction” to predict the actual field performance. The other method introduces the
potential variability of testing two bats that may have different dynamic properties due to
inherent variations in natural wood. The first method is modeled after a player who has a
favorite bat and uses it in different cities and climates. The second method is modeled after
a player who has a number of bats and prefers to choose a bat that gives him the same feel
no matter where he is playing the game of baseball.
SAME BAT METHOD
Fig. 3 shows results that obtained by testing a 33-inch ash baseball bat. The surface of this
bat is unfinished--allowing the moisture to penetrate the surface with very little resistance.
Prior to the test, the bat had been stored in a room kept at 50% relative humidity according
to standard protocol. The bat was then stored in an environment that averaged 85% R.H. for
17 days. The Baum Hitting Machine, which swings both the baseball bat and the baseball
into the hit and measures the incoming velocities of the bat and the pitched ball and the
batted-ball velocity, was then used to investigate the batted-ball velocities at the three
standard test positions (as denoted in Fig. 1) on the bat. The bat was subsequently allowed
to dry in the room kept at 50% R.H. and was tested again using the Baum Hitting Machine at
the same three positions.
The tests were performed on the wooden bat while it had moisture contents of both 10.9
and 6.7%. The moisture content was recorded as the average of the measurements taken on
the barrel of the baseball bat. Fig. 3 shows the averages of the batted-ball velocities as
measured at the 6-foot location from impact for each of the three positions tested on the bat.
The weight is less for the test specimen with less moisture. Therefore, the bat with a
moisture content of 6.7% can actually be swung in the field with a higher velocity than the
bat with a mo isture content of 10.9%. Therefore, an adjustment was made for the swing
speed taking into consideration the change in moment of inertia (Fig. 3). Assuming that the
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moment of inertia introduced its most significant effects when the bat is fully extended and
rotating primarily about the center of the grip of the baseball bat and that there is
conservation of energy, the change in swing speed can be approximated using eq. 1;
? dry = ((Imoist * ? moist 2 )/Idry)1/2
(1)
96
10.9% moisture content
6.7% moisture content
adjusted for swing speeds
6.7% moisture content
Batted-ball velocity (mph)
95
94
93
92
91
5.5
6.0
6.5
Position from barrel end of bat (in)
Fig. 3 Performance of a wooden bat tested at different moisture contents
where ? dry and ? moist are the angular velocities of the dry and moist bats, respectively, and
Idry and Imoist are the mass moments of inertia (MOI) about the position 6-inches in front of
the knob of the bat for dry and moist bats, respectively. The MOI for the moist sample was
11,313 [oz-in 2 ]. The dry sample had an MOI of 11,001 [oz-in 2 ]. Because the baseball bat
speed for the moist sample was 66 mph, the resultant bat speed for the dry bat swung by the
same player would be 66.9 mph. This increase of 0.9 mph would generally lead to about a
0.75 mph increase in batted-ball velocity. Therefore, the dry sample has been graphed with
the 0.75-mph offset to compare the change in performance as predicted by the machine
setup for this particular bat being used after having been exposed to a different climate.
Prior to the adjustment for swing speed, the baseball bat with the higher moisture
content had a distinct increase in performance over the “dry” bat. With the adjustment for
swing speed, the results show that the bat with the higher moisture content has a higher
performance by as much as 1% along the tested region.
SAME WEIGHT METHOD
The second method for investigating wood bat performance was performed in a similar way
to the first method. A bat was stored in an environment that was maintained at an average of
85% R.H. for a 17-day period. It was then tested at the 5.5, 6.0 and 6.5-inch locations.
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Then another bat, which had been stored in the standard lab environment of 50% R.H. and
had a similar MOI and weight, was tested at the same three positions. The results from these
two tests can be seen in Fig. 4.
The results from testing moisture content using this same-weight method are very
similar to the results obtained by the same-bat method. Because both of the bats used in the
same-weight testing have essentially the same MOI, they will be swung with the same
velocity. Therefore, there is no need for any adjustments due to differences in swing speed
to be made to the data. This combination of bats shows the bat with a 10.9% moisture
content to have as much as 1% increase in performance over the drier bat with a 6.7%
moisture content (Fig. 4).
94
Batted-ball velocity (mph)
10.9% moisture content
6.7% moisture content
93
92
91
90
5.5
6.0
Position from barrel end of bat (in)
6.5
Fig. 4 Performance of bats with same MOI but different moisture contents
EFFECTS OF WORKHARDENING
Players and bat manufacturers commonly argue that the performance of aluminum baseball
bats deteriorates with increased use. Theories of stress, strain and workhardening predict a
conflicting result. Workhardening can occur when a metal is deformed beyond its elastic
limit. The deformation, on the microstructural level, results in dislocation generation and
movement, and results in a stronger metal from a yield point perspective on the macroscopic
level. As a baseball bat surface workhardens, more elastic energy is stored in the bat that
can be transferred to the batted-ball, giving the ball more energy than the non-workhardened
bat and hence, the workhardened bat exhibits an increased performance.
The NCAA requires each length/weight/model combination of bat to be tested for
compliance with its bat-performance rule. In 1999, the NCAA mandated that non-wood bats
could perform no better than 34-in. bats made of northern white ash. The UMLBRC is the
official certification center for such compliance testing. Thus, manufacturers send new
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production bats to the UMLBRC for certification, and the NCAA submits bats pulled from
field service to test for compliance. If a given length/weight/model bat combination
workhardens as it is used during a season, then that combination could potentially pass the
certification process, but exceed the established performance limit during the season.
Therefore, the NCAA is interested to know the effects of workhardening of aluminum
baseball bats.
The test method used for measuring the performance of an aluminum baseball bat as a
function of repeated hitting involves performing consecutive certifications on a single
baseball bat. Each certification includes five valid hits at all five positions on the barrel end
of the bat according to standard protocol. Such testing on the hitting machine can be
considered to be representative of field-service use.
Each data point plotted in Fig. 5 represents the average of all 25 valid hits during
certification of a standard 33-inch aluminum bat made of C405 aluminum. The data are
generated from the 9-inch and 13-inch measurements and are normalized based on a
comparison to the pass/fail limit used for the certification of baseball bats. This comparison
is necessary because different lots of baseballs were use throughout the tests. A total of 830
hits were taken on the bat before it was ultimately destroyed due to the appearance of large
cracks in the barrel. A fifth-order polynomial fit of the data is included in the plot to show
the trend of the data. The data imply that the most significant performance increase occurs
near the beginning of the bat’s life (after about 120 hits). The dashed lines indicate several
notable visual changes in the bat’s surface. At about 600 hits, stress marks appeared on the
surfaces that are between the 5.0 and 7.0-inch marks. The appearance of these stress cracks
corresponds to a secondary increase in performance. As the bat was approaching 770 hits,
Note: Each data point represents
the average of 25 valid hits, 5
valid hits at each of the 5 positions
Normalized Average Batted-Ball Velocity (mph)
95.5
95.0
94.5
94.0
stress marks
appear on
surface (~600)
93.5
cracks form
on two sides
of the barrel
(~770)
93.0
92.5
0
200
400
600
800
Hits on Bat
Fig. 5 Performance of aluminum baseball bat through repeated hitting
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1000
the surface on two sides of the bat had cracks running axially. This observation coincides
with the beginning of the steep decline in the bat performance. Overall, the performance of
the work-hardened aluminum baseball bat showed an increase of about 2% from its original
performance.
Workhardening may not always occur. Another bat was tested under a similar process.
After 147 hits on the 33-inch baseball bat constructed of C555 aluminum, the surface had
dented so significantly that testing could not continue. Up to that time, the bat’s
performance had decreased slightly, and the Rockwell hardness had also decreased. The
results from these two workhardening tests identify that workhardening may increase the
performance by about 2% if the bat remains in good condition for a significant number of
hits.
CONCLUSIONS
Both moisture content and workhardening can have an effect on the performance of baseball
bats. The same-bat method and same-weight method used to determine the effective change
in performance from moisture content both showed an increase with a maximum of about
1% when the moisture content went from 6.7% to 10.9%. The result of the workhardening
of an aluminum baseball bat identified that an increase of about 2% in performance is
possible if the bat remains in good condition through a significant amount of use.
REFERENCES
Forest Products Society (1999) Wood Handbook: Wood as an Engineering Material, 12-4
NCAA PROVISIONAL STANDARD FOR TESTING BASEBALL BAT PERFORMANCE
(1999) http://www.ncaa.org/releases/miscellaneous/1999092901ms.htm
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